1. Field of the Invention
The present invention is directed to the production of rare earth-containing frits having a glass transition temperature of &gt;525.degree. C. and to the use of these frits for the production of enamels having improved heat resistance.
2. Description of the Prior Art
Currently available "pyrolytic" enamel systems exhibit crazing and/or flaking (hereinafter referred to as "failure") during pyrolytic oven self-cleaning cycles. These cycles may involve rapid (approximately 25.degree.-30.degree. C./min) temperature gradients to a final temperature of 500.degree. to 525.degree. C. for several hours followed by a rapid cool to ambient temperature. Typical lifetime testing involves 120 such thermal cycles representing a 10 year oven lifetime. Failure is especially evident in dry powder "electrostatic" porcelain enamel systems, as compared to conventional wet systems, which are much more flexible due to the availability of mill additions to tailor thermal properties. For dry powder systems, the thermal properties of the required enamel must be designed into the composition of the glass or glasses which comprise the system.
During the firing process of an enamel/steel composite, the enamel is transformed from a particulate compact to a relatively dense, monolithic coating due to the action of heat. The densification occurs by viscous flow. During cooling from the firing temperature, compressive stresses develop in the enamel due to its lower thermal expansion below the glass transition temperature than the steel substrate. Because ceramics and glasses fail almost exclusively in tension, high stresses are a necessary requirement for a craze-free coating.
Failure in pyrolytic coatings occurs if the pyrolytic cleaning temperature is above the glass transition temperature of the coating. The glass transition is defined as the temperature range when the viscosity of the glass is approximately 10.sup.13 poise. In practical terms it is the temperature range where the physical properties of the glass change from that of an elastic solid to a viscoelastic material. Above the glass transition temperature viscous deformation will occur in response to an applied stress. This means that at or above the glass transition temperature, the glass comprising the coating will relax, the compressive stress will diminish and eventually during subsequent cleaning cycles the coating will be placed in tension, thereby increasing the probability of crazing. Another result of the coating experiencing a temperature higher that its glass transition temperature is general thermal degradation of the coating including discoloration, crystallization, iron oxide penetration and metallization. This behavior is qualitatively described as thermal durability. All of the aforementioned phenomena are undesirable and may aggravate flaking of an already crazed enamel coating. Flaking occurs when large pieces of a degraded enamel coating physically separates from the metal substrate, leaving the substrate exposed.
It is also known to prepare dry powder "pyrolytic" compositions by blending two or more frits or glasses which each possess certain desirable characteristics. For example, a typical composition may contain 50% by weight of a "soft, bonding" glass, i.e., a glass with a low glass transition temperature and low viscosity at the firing temperature which readily adheres or bonds to the metal substrate, and 50% by weight of a "hard, pyrolytic" glass, i.e., a system with a relatively high glass transition temperature which possesses excellent thermal durability , but usually exhibits little or no bond. Typical glass transition temperatures for the bonding glass may be 450.degree. to 480.degree. C., while the glass transition temperature for the pyrolytic glass is 480.degree. to 510.degree. C. The problem with this approach is that during the self-cleaning heating cycle, one or both of the glasses are exposed to a temperature which is greater than their respective glass transition temperatures, which leads to failure as previously discussed.
Accordingly, it is an object of the present invention to develop a frit which may be applied by the dry powder electrostatic process to form an enamel which is suitable for use as a coating for self-cleaning ovens and which does not craze or flake during repeated self-cleaning cycles. It is an additional object of the present invention to provide a frit which may be fired at a temperature of 810.degree. to 840.degree. C. for 3 to 10 minutes to form a coating which has densified and matured sufficiently to yield a smooth continuous coating. It is a further object of the present invention to provide a frit which is suitable for the preparation of coatings which possess excellent acid resistance, good to excellent bond on pickled extra low carbon steel and also exhibit acceptable color and gloss characteristics.
These objects may be achieved in accordance with the present invention as described hereinafter.